Vaulted Clipped Hip Roof

[BS2]

I am analyzing an existing wood shingle roof that is being converted to slate. The roof configuration is a vaulted clipped hip (for lack of a better name) - see attached. The original construction, as far as I can tell, is basic stick framing with no special detailing for exotic framing methods (bent steel beams, tension rings, scissor trusses, etc).

I have analyzed the framing in RISA and the analysis indicates that the existing framing is adequate with the addition of many nails at the ceiling joist/rafter connection, and braces to provide stability to the structure. These items are shown on the attached sketch. So far this analysis does not take into account the beneficial effects of the sheathing on a steep roof such as this. Any advice on how to account for such effects would be appreciated.

With the addition of this much weight, I wanted some additional opinions on whether my proposed solution seems reasonable. Has anyone ever encountered a framing solution similar to this, and if so, what was your solution?

Thanks!

 

[Archie264]

Does your model show the lateral thrusts at the base of those rafters? It looks like it could be significant and might push out the walls considerably. Make sure to investigate that.

 

[BS2]

Thank you for the response!

I have modeled it with a pinned connection on one end and a roller on the other. See attached. The extra boundary conditions are to keep it stable in the model.

In the model, the lateral deflection of the right rafter is 1.16" whereas the left support does not move. Removing this idealization and dividing this lateral movement between the two sides results in 0.58" per side. I am not sure if this acceptable or not. However, it is much better than what I initially thought it might be. Additionally, the sheathing may help some with the deflection.

 

[KootK]

In my experience, these sketchy hand framed roofs stay up because the diagonal ridge beams form pseudo-tied arches with the plywood acting as the tension ties. The rest of the framing spans from the walls to the ridge beams.

That being said, I don't see you getting any reliable, quantifiable help from the sheathing in this situation. And I wouldn't know of an expedient way to model that behaviour anyhow. In addition to Archie's concern:

1) She might come down when you remove that existing sheathing as you'll interrupt that tied arch system and;
2) Watch for an unbalanced case that may occur when the tile is installed on one side of the roof but not the other.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

Have you modeled it for wind?

BA

 

[woodman88]

I think that there are to many nails into the 2x8 at the ceiling/brace point. I would look at using verticals at the top of the 2x8 down to the ceiling tie. with a diagonal between the verticals. If the numbers work.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[XR250]

I would worry about long term creep in this situation. 6/10" per wall is pretty significant and it will only get larger over time with the heavy load of the slate.

 

[KootK]

@BS2: Those 2x at the pitch breaks. Blocking pieces or continuous through the 2x8 rafters?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

It would help if the ceiling could be lowered a bit.

BA

 

[BS2]

Thanks for all of the responses. I will try to address all of the great comments.

@KootK: Currently there is skip sheathing which the contractor would like to leave in place. They would like to add 1/2" OSB sheathing on top of the skip sheathing.

I haven't checked the unbalanced case yet. I will go do that right now.

The 2x8's (or they could be 2x10's, I am not sure at the moment) at the flat roof transition are continuous through the rafters.

@BAretired: I just checked it for wind and it works ok. The deflection is approximately the same as the gravity load case. To account for the wind load on the wall, I used a point load acting on the bottom of the rafter. This load would run through the roof system diaphragm and into the shearwalls.

I am considering telling them they need to lower the ceiling. I have modeled it with the ceiling dropped approximately 24" and the stresses/forces are much more reasonable. In addition, the deflection is 3/4" total or 3/8" on each side, before accounting for creep.

@woodman88: I was also concerned about the amount of nails required. However, I have not been able to find information on a minimum nail spacing. I believe I have them layed out at 2"oc. Do you have any recommendations for the minimum spacing on nailed connections like this?

Is your comment about adding verticals to eliminate the extra nails from the diagonal braces? If so, I was considering changing this to something similar to what you suggest.

@ExcelEngineering: I am also concerned about the long term creep. This roof has been constructed for many years (at least greater than 10 years - the house is 60-70 years old, but this may be an addition) with no apparent deflection issues. Some creep may already be present, but the additional weight we will be adding will certainly increase the creep.

 

[KootK]

The 2x8's (or they could be 2x10's, I am not sure at the moment) at the flat roof transition are continuous through the rafters.

Thanks for the answers.

1) This lends credence to my original comment that I believe this to be a rafter and ridge beam system rather than wall to wall framing.

2) This will make the two joints at the top of your truss difficult to construct. The ridge beam will be in the way.

3) If you proceed with this scheme, I'd consider just sheathing the truss with plywood. You can omit the webs save those required for sheathing blocking. Glue and screw the plywood. You want this as stiff as it can be.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BS2]

1) I believe that the original construction of this roof had no special detailing for wall to wall framing. It was simply constructed like any other roof rafter system - except for the lack of ceiling joist ties at the eave. I agree with you (KootK) that the only way this is still standing is the hip rafters and sheathing acting as pseudo-tied arches. The ridge beams are not sized to span between hip rafters. However, I believe they act as a combination beam and pinch board.

2) I may be misunderstanding you (and maybe you me). It sounds like you were thinking of a ridge beam spanning between reinforced trusses. Is this correct? If so, I believe the stresses for built up wood trusses would be too high. However, bent steel beams may be a possibility.

My original thought was not adding a ridge beam to the trusses. I was planning on reinforcing every single joist/rafter along the entire roof. The detail attached to my original post shows the reinforcement at each joist/rafter. Of course, the joists/rafters near the top of the hip rafters would be double or maybe even tripled to stiffen up the whole area. I have done this many times (and it works quite well) at hips where it may not be possible to install a beam to support the hip/ridge connection. For example, raised ceiling joists on a hip roof is a great candidate for this technique.

In addition, I was considering adding steel straps at the corners of the top plates and framing clips from the hip rafters to the top plates to encourage the system to act as a tension ring.

As an aside, this house has several other areas with this type of poor framing. In one area, because of an addition, the roof framing was even more complicated and I told them that wood trusses would save everyone time, effort, and money. Maybe I need to do that here also. Although, right now, my pride as an engineer may be getting in the way.

3) I thought of trying plywood. I was hoping to avoid it. The only thing holding me back is the amount of work required to install plywood gussets at each ceiling joist/rafter.

 

[woodman88]

BS2

@woodman88: I was also concerned about the amount of nails required. However, I have not been able to find information on a minimum nail spacing. I believe I have them layed out at 2"oc. Do you have any recommendations for the minimum spacing on nailed connections like this?

Per the NDS (2001) section 11.1.5.6 "Edge distances, end distances and spacings shall be sufficient to prevent splitting of the wood." The problem is that the spacing required changes for density and moisture content of the lumber.

Is your comment about adding verticals to eliminate the extra nails from the diagonal braces? If so, I was considering changing this to something similar to what you suggest.

Yes that is what I meant.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BS2]

It has been awhile, but I wanted to give an update on this problem. I have decided against strengthening the wood rafters. Instead, I am headed down the steel frame path. I have included a portion of the drawings for this project to show you my proposed solution. Note that this is part of a larger structure (not shown).

In your professional opinions, would you consider this an acceptable solution?